By yourself :

This is a bit more expensive but allows you to get out of the rat race of travellers, to gain some time and to be able to choose where you would like to sleep.

In Bangkok, go to the Mochit 2 bus station towards the airport. Air conditioned buses leave every hour towards Aranyapathet-Poipet (170 bahts - 4 hours). From there a tuk-tuk will take you to the border for 50 bahts. Then cross the border on foot. Watch out for picpockets! There are lots! For those who don’t have a visa there is a cashier that does them in a few minutes for 1000 bahts. Avoid intermediaries! Once in Poipet, near roundabout, taxis and pick-ups leave for Siem Reap. They won’t leave until they are full. If you choose a pick-up, you must change along the way at Sisophon, even if the driver states otherwise. (About 300 bahts inside).

PAVILION'S INDOCHINE TIP

The most comfortable way from Poipet to Siem Reap is:

The best way is to rent an entire taxi, usually a Toyota Camri, just after the immigration at the turaround
(split between several travellers if you want) for about 1200 to 1500 bahts.
3 hours travel for the 150 kilometres. The road is so-so condition.

By tour-bus:

It is the cheapest way but also the longest and most tiring to make the journey between the city of Angels and the city of temples. Watch out! Travel agents, notably in Khao San Road, sell bus tickets at ridiculously low prices (80-150 bahts). Be very clear that these prices are hiding something. The driver makes the journey as long as possible and numerous stops fo no reason are made along the road. By this means, the bus arrives at a guesthouse in the middle of the night who has previously set this up. Those who don’t want to stay here often have to insist strongly to be able to leave. The journey is generaly 15 hours.

Usually a bad experience but it is cheap.